This invention relates to ultrasound imaging systems, and in particular to methods and systems for simultaneously displaying image information derived from multiple imaging modes.
Ustuner et al., U.S. Pat. No. 5,479,926, disclose a B-mode image enhancement method for combining a first image signal which has greater detail information and a second image signal which has greater contrast information, into a single image using a 2D look-up table. In one of the preferred embodiments, the second processed image can be obtained from a motion estimator which calculates the correlation coefficients between consecutive B-mode frames.
Arenson et al., U.S. Pat. No. 5,285,788, disclose a Doppler tissue imaging method (DTI) that uses color Doppler imaging means to image moving tissue. The disclosed DTI imaging can output tissue velocity, energy, or acceleration as a two-dimensional image which is spatially coordinated and superimposed on a B-mode image to display simultaneously the selected Doppler information and a tomographic image of the moving tissue. For Doppler tissue velocity imaging (DTV), the moving tissue velocity is the primary parameter to be displayed. Conventionally, a color map is used to encode the direction as well as the magnitude of the velocities, and a gray scale B-mode image signal may also be partially added to provide tomographic information of the moving tissue.
In the past, various contrast agents have been used to enhance contrast of blood and perfused tissues. Typically, a contrast agent is introduced into a part of the body which is to be ultrasonically imaged. For example, in the case of a blood-filled chamber of the heart, blood which carries contrast agent can be distinctly imaged by detecting the contrast agent.
Nonlinear scattering from contrast agents is described, for example, by V. Uhlenhdorf, et al., in "Nonlinear Acoustic Response of Coated Microbubbles in Diagnostic Ultrasound" (1995 Ultrasonic Symposium, pp. 1559-1562). Such contrast agents possess a fundamental resonant frequency. When the contrast agents are insonified with a high intensity ultrasonic energy at this fundamental frequency, they reflect and radiate ultrasonic energy at both the fundamental frequency and a harmonic of the fundamental frequency. For example, if insonified at a frequency of 2.5 MHz, the contrast agent may radiate energy at both 2.5 MHz (the fundamental frequency) and at 5.0 MHz (the second harmonic frequency).
Typically, non-linear contrast agents are used with an imaging system having a transmit beamformer that transmits ultrasonic energy and a receive beamformer that receives the reflected ultrasonic energy. The transmit beamformer insonifies the area to be imaged with ultrasonic energy at a fundamental frequency. When insonified with ultrasonic energy at the fundamental frequency, the contrast agent radiates energy at both the fundamental and harmonic frequencies as described above. The receive beamformer receives both the fundamental and harmonic energy, filters out the fundamental energy, and forms a harmonic image from the received harmonic energy. Ideally, the harmonic image relates only to the scattering from the contrast agent.
The harmonic image, however, may contain harmonic frequency components related to scattering from tissues in addition to the desired harmonic energy. For example, the transmit beamformer may transmit energy at the harmonic frequency as well as at the fundamental frequency. This energy scatters linearly and is included in the harmonic image. In addition, the receive beamformer may not completely filter out energy at the fundamental frequency, so this fundamental frequency leaks into the harmonic image. Finally, non-linear scattering from tissues or non-linear propagation through tissues may result in harmonic energy being scattered from normal tissues and included in the harmonic image, even in the absence of a contrast agent.
Brock-Fisher et al., U.S. Pat. No. 5,577,505, combine a colorized non-linear image with a gray-scale image. The non-linear image is obtained via a subtraction approach, requiring insonifing the tissue at two different times and power levels. Further, the combination includes only the simple steps of colorizing the non-linear signal and summing with the gray-scale image.
Monaghan, U.S. Pat. No. 5,255,683, combines a B-mode image taken before the introduction of a contrast agent with a subtraction image formed from images taken after a contrast agent has been introduced. Monaghan, however, requires images to be acquired before and after the introduction of a contrast agent. The scan is thus not in real-time, and the scan plane must be identical for each firing before and after the introduction of the non-linear contrast agent.
Conventional ultrasonic imaging system combine B-mode imaging modes and color Doppler imaging modes. Such systems usually have distinct signal paths for the B-mode and Doppler signals. Since different transmit and receive beams are required for regular B-mode imaging and color Doppler imaging, acquisition time is typically shared between the two modes. A typical acquisition sequence is to acquire one complete frame of B-mode information and then to acquire one complete frame of color Doppler information, alternating between the two modes. This method is conventionally referred to as "frame interleaving". Another typical acquisition sequence is to alternate between the two modes of imaging in a line by line sense, i.e., one line of B-mode information is collected followed by one line of color Doppler information. This mode of operation is called "line interleaving". Other modes of operation between these two interleaving techniques are also possible, i.e., many color Doppler lines may be collected between every pair of B-mode lines.
In the past, various legends have been displayed to inform the user of the mapping function currently being used to map measured signals to output image signals. For example, with conventional B-mode imaging, a one-dimensional legend that identifies display indicia such as gray-scale level with the range of values of the output B-mode signal has been displayed. Similarly, it is known to display a two dimensional legend that defines the display indicia of a mapping function for mapping two Doppler images (such as a color velocity image and a color energy image) into the displayed output image. Alternately, Doppler velocity and variance in Doppler velocity have been used as two axes of a mapping function that is displayed as a two dimensional legend.
Also, it is known to allow a user to select mapping functions in two stages, first by family of mapping functions, and then by individual mapping function contained within a previously selected family. Such techniques have been used in Doppler imaging systems where, for example, Doppler velocity and Doppler energy may be mapped to an output image, and where the user has been provided with means for selecting a mapping function family and an individual mapping function contained within a previously-selected family.